A Hydrex diver/technician team flew to Pointe-Noire, Congo to replace the stainless steel belt in a research ship's thruster tunnels.
The stainless steel belt is installed around the perimeter of a thruster tunnel at the location of the thruster blades. There the impact of the cavitation caused by the movement of the blades is the most severe. Extra protection against cavitation damage is therefore essential. When the stainless steel belt in the thruster tunnel of the research vessel suffered cracks, the underlying steel was exposed to cavitation. The belt needed to be replaced as soon as possible to prevent the thruster tunnel from getting damaged too severely. The owner of the vessel would have had to take his vessel to drydock if no on-site solution was found.
A tailor-made open-top cofferdam was designed by the Hydrex technical department. It was constructed in a local workshop in Pointe-Noire under the supervision of Hydrex diver/technicians. At the same time a regular shaped second cofferdam was also built. Cofferdams like these are used to close off the thruster tunnel on both sides. Water can then be drained from the tunnel, creating a dry working place. This allows the team members to descend into the tunnel through the open top. They can then carry out any necessary welding work in drydock-like conditions.
This prevented an unscheduled drydock visit for the ship and saved the owner precious time and money.
Unfortunately underwater visibility was down to near zero, so the ship was moved to an anchorage at Port Gentil, in neighbouring Gabon, where conditions were more favourable. Here a diving station was set up and the diver/technicians started the installation of the cofferdams. Next they emptied all water from the thruster tunnel. They then descended into the tunnel and carried out the repair. The old, damaged, belt was removed and replaced with a new stainless steel belt.
The actual operation was finished in only five days. The team removed the cofferdams and the ship was ready to continue its schedule with its thruster tunnel fully protected against cavitation once more.